Scheduled Ancient Monument: ATOMIC BOMB STORE ON THETFORD HEATH (30608)

The monument includes a bomb store which was one of two designed and built in the 1950s for the storage and maintenance of Britain's atomic bombs. The storage compound covers an area of around 9ha and is pentagonal in plan, enclosed by a double fence with a patrol path between. Within this enclosure the various storage and maintenance units for the components of the bombs are laid out in a functionally designed landscape. A metalled track leads from Elvedon Road, 30m to the south, to the entrance, and on either side of the gate to the main compound is an outer fenced enclosure containing various ancillary buildings and other structures, chiefly relating to site security. Planning of the facility is thought to have begun in 1952 and the site was purchased by the Air Ministry in 1954. Building work was substantially complete by August 1955 and the station, known as RAF Barnham, became operational at the beginning of September in the following year. It was commanded by No.40 Group and formed part of No.94 Maintenance Unit, with headquarters at RAF Honington, approximately 6km to the south east. It was intended that this group should supply airfields at Honington in Suffolk and Marham and Watton in Norfolk. In the original design the ancillary buildings to either side of the main entrance were situated between the inner and outer fences of the main compound, but at a later date, probably in 1958, the fences here were moved back and the present outer enclosure constructed. The early free-fall bombs for which the storage system was designed became obsolete with the introduction of the stand-off missile Blue Steel in late 1962, and storage of weapons here probably ceased in 1963. The site was sold into private ownership by the RAF in late 1965, but the layout and most of the original structures remain. The bombs originally stored here were large, measuring 7.3m in length by 1.52m in diameter and weighing 4636kg. The two principal components were the plutonium core and the machined lenses of high explosive which surrounded the core and, when detonated, would cause it to implode to form a critical mass. The fissile cores were stored separately from the outer casing containing the explosive lenses and the electronic and mechanical components. These bombs required careful maintenance and monitoring. Some of the nuclear components were highly unstable and the cores containing them had to be reassembled at frequent intervals. The high explosives also had to be kept in a controlled environment. The track giving access to the storage area is enclosed by a wire fence carried on concrete posts. Entry, at the southern end, was controlled by double gates, and immediately behind these, to the right of the track, is a picket post for an armed guard. Beyond the picket post, also to the right of the track, are bays for a motor transport shed, fuel compound and two standby generator buildings. The track runs due north, then bends eastwards at right angles towards the gate of the outer enclosure. The metal rails for the original electric inner gate, moved back when this side of the storage compound was remodelled, can be seen in the surface of the track. Most of the buildings within the outer enclosure are of prefabricated construction. On the east side of the track, immediately to the right of the original gate, is a hut which housed the duty officer and a small brick building which was the standby generator house, with a three sided brick structure behind it to contain a transformer and sub-station. To the east of these is a small brick building which was the small arms ammunition store. Buildings on the opposite side accommodated the RAF Police dog section. Alongside the track is an L- shaped hut which contained the guard room and a control centre, with a small mess and sleeping accommodation for the dog handler, who was the only person billeted on site. Behind this is a small building used as the meat preparation store for the dogs, and beyond it, in the angle of the outer enclosure, the concrete base of the dog compound, on which the outline of the individual runs can be traced. Other buildings on this side include the former fire station, the telephone exchange and a gym. In the southern angle of the enclosure is a water tank and pump. The main enclosure is aligned on a north east-south west axis, with the entrance at the south western angle of the pentagon. The two fences are set about 22m apart, and the area between was lit by lamps on concrete posts. The outer fence is of wire mesh supported on concrete posts which are angled outward at the top, and a perimeter path runs around the inner side. At the mid point along the north west, north east and east sides the fence line is offset to form a triangular bastion about 5.2m wide and 1.5m deep, allowing patrolling guards to sight along the external faces of the fence. At the western, northern, north eastern and south eastern angles there are steel framed observation towers 8.2m high with roofed cabins on which search lights were mounted, and there is a fifth observation tower inside the fence, immediately to the east of the entrance gate. The inner fence is of concrete panels slotted into concrete uprights. Immediately to the west of the inner gate which is set in this inner fence, is a small building thought to have been a picket post. The storage and maintenance buildings radiate from a sub-oval circuit road around a central area enclosed by an earthen bank. The three buildings to house the non-nuclear components of the bombs were almost identical in plan and were positioned symmetrically, one at the north eastern end of the circuit and one on either side. The building on the north western side was demolished following a fire in the 1980s, leaving only the concrete floor slab, but the other two survive. All three are surrounded by earthen banks 4.42m high revetted by reinforced concrete walls at the ends abutting the roadway. The two surviving buildings are constructed of reinforced concrete columns and beams, with walls of precast concrete blocks, and are rectangular in plan, measuring 57.97m by 18.29m internally. Two longitudinal rows of columns support the reinforced concrete roof slabs, dividing the interior into a central corridor 7.62m wide with aisles of 11 bays on either side. Originally these bays were open, but dividing walls have been inserted to convert them into workshops. The floors, which are included in the scheduling, are surfaced with a patented gritless compound to reduce the risk of accidental sparks. The access doors at the end of the buildings opening onto the road are 3.05m wide and 3.66m high, and there are emergency exits at the opposite ends. Flanking the entrances are buildings which contained the heating and air conditioning plant for the store. The bombs were loaded and unloaded by means of a gantry extending above the roadway, with reinforced concrete support columns and beams, to which were attached a steel joist runway beam, originally fitted with a hoist. The fissile cores were stored in 54 small, rectangular, kiosk-like buildings or `hutches' arrayed in four groups between and to the south of the non-nuclear component stores. The buildings in each group are linked by a rectilinear system of footpaths leading off a primary path which opens off the main road circuit. The paths are fenced by tubular steel railings and where the ground rises, in the north eastern part of the site and at the entrance to the path to the south western group, they are cut level into the surrounding surface. The areas were lit by concrete lamp posts. Free standing walls, set around the central area within the main road circuit, face the entrances to each of the path systems. The buildings themselves are constructed of concrete, with foundations 0.91m thick, cavity walls, and flat, overhanging roofs covered with bituminous felt. The walls are rendered internally with gritless plaster, with four small ventilators inset at the sides and to the rear. The doors are of wood protected externally by a steel sheet, and are secured by a combination lock and internal, vertical locking bar operated by an external handle. Attached to the frame above the doors were spring loaded electrical contacts, probably to signal whether the doors were open or closed to an external control board at the gate. Many of the buildings retain the remains of other electrical fittings, including small bore pipes, junction boxes and switch boxes. The fissile cores were contained in stainless steel vessels sunken into the floors The vessels have been removed but the shafts into which they were sunk remain. These are cylindrical, about 0.54m deep and 0.44m in diameter, with a shallower, rectangular depression 0.09m deep and measuring 0.27m wide by 0.21m at the lip, forming a keyhole shape. The buildings are of two types. The majority measure 2.54m by 2.39m in plan and have a single shaft in the centre of the floor. Nine of them, grouped in threes in the northern, north eastern and south western arrays respectively, are slightly larger, with two shafts in the floors. It is thought that the single shafts were for plutonium cores, and the double shafts for cobalt cores. At some stage the double shafts were sealed with asphalt, but this has been removed and the shafts re-exposed in one of the buildings in the northern array. A small brick building shielded on three sides by a brick wall and situated alongside the path leading to the northern array is thought to have been used for the inspection of cores. The other maintenance buildings are situated opposite to or near the entrance to the main enclosure and accessed by bypass loops off the main circuit road. Immediately opposite the inner gate is a rectangular building constructed of concrete blocks, with a porch and self contained battery charging room at the front. The original building has been extended to the east. Behind this, and separated from it by an earthen bank, is a larger and taller building of similar construction to the non-nuclear component stores, with air lock porches at either end and separate plant rooms and a dark room to the rear. This was probably used for the assembly or maintenance of warheads, and the entrances are shielded by high, free standing breeze block walls. Another large building of brick stands to the west of the entrance. This was constructed in 1959 and is described in the design drawings as `Inspection and Repair Workshop'. Other features of the inner enclosure include three large, static water tanks located one on either side of the north eastern non-nuclear component store and one to the west of the south eastern array of fissile core stores. A number of features are excluded from the scheduling, these are: the wire mesh of the fences, wooden fences, fence posts which post-date the occupation of the site by the RAF, service poles in use and supports for signs post-dating the occupation of the site by the RAF, an electrical substation in use, inspection chambers in use, fuel tanks, the superstructure of the non-nuclear component stores and the associated gantries and all other roofed buildings other than kiosks for the storage of the fissile cores. The original floors of the non-nuclear stores, the ground plans of the excluded buildings and the ground beneath all these features are, however, included.